1. Field of the Invention
This invention relates generally to the fabrication of thin films by ion beam sputter deposition and, more particularly, to the fabrication of multilayered thin film structures such as magnetoresistive sensors wherein the thickness uniformity of multiple layers deposited on a substrate are controlled by controlling the atom flux deposited on the substrate.
2. Description of Related Art
It is well-known in the prior art to utilize RF or DC magnetron sputter-deposition apparatus for fabrication of thin film devices such as magnetic recording sensors and storage media. Such sputter devices are characterized by crossed electric and magnetic fields in an evacuated chamber into which an inert, ionizable gas, such as argon, is introduced. The gas is ionized by electrons accelerated by the electric field, which forms a plasma in proximity to a target structure. The crossed electric and magnetic fields confine the electrons in a zone between the target and substrate structures. The gas ions strike the target structure, causing ejection of atoms that are incident on a workpiece, typically a substrate on which it is desired to deposit one or more layers of selected target materials.
In the prior art conventional sputtering devices relatively high operating pressures are utilized in order to obtain films having low internal stress which results in non-directional sputtering flux at the substrate. However, this non-directional flux introduces manufacturing process difficulties as device dimensions become increasingly smaller.
It is known in the prior art to utilize ion beam sputter deposition in certain applications to overcome some of the difficulties encountered with conventional RF/DC sputter techniques. Several aspects of ion beam sputter deposition differ from conventional sputter processes and provide significant advantages. For example, (1) the use of low background pressure results in less scattering of sputtered particles during the transit from the target to the substrate; (2) control of the ion beam directionality provides both a variable angle of incidence of the beam at the target and a controllable angle of deposition at the substrate; (3) a nearly monoenergetic beam having a narrow energy distribution provides control of the sputter yield and deposition process as a function of ion energy and enables accurate beam focusing and scanning; and (4) the ion beam is independent of target and substrate processes which allows changes in target and substrate materials and geometry while maintaining constant beam characteristics and allowing independent control of the beam energy and current density.
Apparatus and methods for producing a thin film deposit on a substrate utilizing ion beam sputtering are described, for example, in U.S. Pat. No. 4,923,585 ('585) to Krauss et al. and in U.S. Pat. No. 5,942,605 to Pinarbasi ('605), the contents of which are incorporated herein by reference. Krauss et al discloses the use of a computer controlled, single ion beam with a quartz crystal monitor to produce deposited films of arbitrary composition as well as layered structures of arbitrary thickness from multiple targets of elemental components of the desired films and layered structures. Pinarbasi discloses matching the ion beam gas atomic mass to the target material atomic mass to provide thin films having densities and physical properties very close to their bulk property values. Both the mass of the ion beam sputtering gas and the energy of the ion beam is controlled as a function of the target material to provide single-layered and multilayered structures wherein selected properties of each layer are optimized to provide specific function for each layer in the resulting structure. While the '585 and '605 patents disclose methods for depositing multilayer films and controlling the physical properties, the problems of controlling and improving the thickness uniformity across the wafer of each individual layer deposited are not addressed.
Ion beam sputter deposition systems have been utilized to deposit individual layers of anisotropic magnetoresistive (AMR) sensors and giant magnetoresistive (GMR) sensors for use in magnetic recording devices. In the GMR sensors, for example, the resistance of the magnetoresistive (MR) sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between the ferromagnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the ferromagnetic and non-magnetic layers and within the ferromagnetic layers. GMR sensors using only two layers of ferromagnetic material (e.g., NiFe or Co or NiFe/Co) separated by a layer of GMR promoting non-magnetic metallic material (e.g., copper) are generally referred to as spin valve (SV) sensors.
Magnetoresistive (MR) sensors (AMR or GMR) are very small devices that are generally fabricated by sputtering depositions on large wafer substrates which are generally larger than 5 inches in diameter to form thousands of sensors that will be subsequently separated (diced) to form individual magnetic read transducers for use in magnetic storage devices.
One of the critical issues in the fabrication process of MR sensors is the thickness uniformity of each and every deposited layer over the entire utilized area of the wafer in order to control the uniformity of operating characteristics (for example, resistance and magnetoresistance) of the entire batch of the MR sensors fabricated on the wafer substrate.
In an experiment by the present applicant, an ion beam sputtering system (FIG. 1) having a non-movable flux regulator was developed and used to build SV sensors 200 (FIG. 2) on a 5 inch diameter wafer substrate (FIG. 3) in order to measure the uniformity of physical properties of the various sputtered layers (therefore, uniform thickness).
Referring to FIG. 1, there is shown a simplified diagram illustrating the ion beam sputter deposition system 120 developed and used by the Applicant. The ion beam sputter deposition system 120 includes a vacuum chamber 122 in which an ion beam source 121 is mounted. The ion beam system 120 further comprises a target 123 and a workpiece (also referred to as a deposition substrate or wafer substrate) 131. An ion beam 133 provided by the ion source 121 is directed at the target 123 where the impacting ions cause sputtering of the target material. System 120 further included selectable multiple targets 123 on a rotary target support 125. The sputtered atoms 126 emitted from the target material is directed onto a deposition substrate 131 on which is formed a layer of the target material.
A thickness monitor 137 positioned closely adjacent to the substrate 131 provides real-time, in-situ monitoring of the thickness of the growing film during deposition. A non-movable flux regulator 150 fixed in front of the deposition substrate 131 partially blocks the sputtered atom flux and is used in conjunction with rotation of the deposition substrate 131 to improve thickness uniformity of the deposited layer. Applicant's non-movable flux regulator refers to a flux regulator that its positioned was fixed prior to the ion beam sputtering deposition of one or more deposited layers and never changed during the deposition of said one or more deposited layers. The substrate or other workpiece 131 is mounted on a movable pedestal or support member 141 which is retrieved into a loading port 139 via a gate valve 138 for changing the workpiece 131. A magnetic field may also be applied at the workpiece 131 if required for the particular structure being deposited. The pedestal 141 may also be rotated, using a rotary/linear motor, during deposition to rotate the deposition substrate 131. During operation of the ion beam sputter deposition system, the vacuum chamber 122 is maintained at suitable low pressure by a vacuum pump (not shown) via port 135.
FIG. 2 shows the SV sensor 200 manufactured using Applicant's ion beam sputtering system 120. SV sensor 200 comprises end regions 204 and 206 separated by a central region 202. A free layer (free MR layer, free ferromagnetic layer) 210 is separated from a pinned layer (pinned MR layer, pinned ferromagnetic layer) 220 (which may include a very thin Co interface layer 218) by a non-magnetic, electrically-conducting spacer layer 215. Alternatively, pinned layer 220 may be made of multi layers of ferromagnetic material (e.g., cobalt, Nife) separated from each other by a metallic non-magnetic conductor (e.g., ruthenium). Such a multi-layer pinned layer is generally referred to as anti-parallel (AP) pinned layer. The magnetization of the pinned layer 220 is generally, although not necessarily, is fixed through exchange coupling with an antiferromagnetic (AFM) layer 225. The AFM layer 225, is generally made of NiMn, FeMn or NiO. The magnetization of the free layer, however, is free to rotate in response to an external field. Free layer 210, spacer layer 215, pinned layer 220 and the AFM layer 225 (if used) are all formed in the central region 202 over the substrate 228. Hard bias layers 230 and 235 formed in the end regions 204 and 206, respectively, provide longitudinal bias for the MR free layer 210. Leads 240 and 245 formed over hard bias layers 230 and 235, respectively, provide electrical connections for the flow of the sensing current Is from a current source 260 to the MR sensor 200.
FIG. 3 shows a wafer 300 manufactured by Applicant's ion beam sputtering system for making SV sensors 200. FIG. 3 illustrates schematically the general pattern of several blocks 301, each block comprising a plurality of rows 302. Each of the rows 302 comprises a plurality of SV sensors 200 disposed along each row and formed on the wafer substrate 306.
As mentioned earlier, Applicant conducted an experiment in which the ion beam sputtering system 120 was used to build SV sensors 200 on the wafer substrate 306. In doing so, layers of sputtered material comprising the layer structure of the central region 202 of the SV sensor 200 were individually deposited and their uniformities were measured by measuring the sheet resistance of the film at the five positions 305 indicated on the five inch diameter of the wafer 300. The uniformity of the sheet resistance which is representative of the film thickness uniformity across the wafer can be expressed as a percent uniformity. The percent uniformity is a measure of the maximum variation of film thickness that will be seen on a given wafer.
FIG. 4 is a graph showing the normalized sheet resistance of the Cu spacer film 215 of the SV sensor 200 measured at five positions 305 across the 5 inch diameter wafer 300. For the data of FIG. 4, the Cu film thickness varied by as much as 11.3% across the wafer 300. The sheet resistance of the NiFe and Co layers of the SV sensor 200 were also measured at the same five locations 305 across the wafer 300. It was observed that the NiFe and Co layers' thicknesses varied by about 3.5% and 2.7%, respectively.
In MR sensors the thickness of each of the thin film layers in the multilayer structure is critical to proper operation of the sensor. Variations of 11.3% in the thickness of the Cu spacer film across the wafer means that many of the MR sensors on wafer 300 fail to work properly or have unacceptably large variations in their responses. Also, as the size of the wafer increases to improve productivity, the problem of achieving film uniformity across the wafer becomes worse. The prior art does not address the need for control of the thickness uniformity of the individual thin layers deposited over the entire area of the wafer to ensure the desired device performance for an entire batch of magnetic devices fabricated with an ion beam deposition system.
Therefore, there is a need for an invention of a method and apparatus for controlling the thickness uniformity of individual films of a multilayer thin film structure deposited on a wafer substrate in an ion beam sputtering system.
There is a further need for an invention disclosing a method and apparatus to control the thickness of individual layers of the multilayer structures of MR sensors deposited on a wafer substrate in an ion beam sputtering system.